The carrying capacity is the rough limit for populations to sustain their own species. Natural selection will be the cause for populations to fluctuate around their carrying capacity. Just think of the simple predator and prey model. less predators + excessive prey = decrease in prey.
As a sidenote, I think the only species that has exceeded their carrying capacity are humans. Since we have managed to extend our natural life by many years, we find the factor that can limit our carrying capacity, and we find ways to either live with it, or eliminate it. Cancer and AIDS are a couple ways, that nature is managing to limit the growth of our population. Without these diseases, we will in fact exceed our carrying capacity (if we haven't done so yet)